The present invention relates to a process for the recovery of mercury and arsenic contained in a hydrocarbon charge, the majority of the mercury being in elementary form and/or in the form of mercaptides. The process is characterised in that the charge is placed in contact with at least one recovery mass M1 for the mercury at a temperature T1 of less than 175.degree. C. and that the product obtained is placed in contact with a recovery mass (M2) for arsenic in the presence of hydrogen at a temperature T2 which is greater than, or equal to, T1 and which is above 130.degree. C.
Mercury and arsenic are pollutants which are often present in hydrocarbon cuts such as liquified petroleum gases, and condensates associated with natural gas or naphthas. Valorisation of these hydrocarbons in various refining and petrochemical processes is impeded because the installations and catalysts become contaminated. Steam-cracking can be cited as an example, where the treatment of hydrocarbons polluted by mercury and arsenic can cause both corrosion of the aluminium alloy based exchangers in the cryogenic section used for separations, and poisoning of the selective hydrogenation catalysts of the olefinic cuts produced.
In the domain of the refining of crude petroleum, similar contamination problems, such as contamination by arsenic, can be encountered, which leads to poisoning of the hydrotreatment catalysts.
Prior art known for the removal of mercury or arsenic from hydrocarbons includes assignee's process described in U.S. Pat. No. 4,911,825, allowing mercury and arsenic contained in a hydrocarbon to be recovered. This process comprises treatment in two successive steps of different kinds of masses, also under different operating conditions.
The recovery mass for arsenic used in the first step is constituted of at least one metal M selected from the group formed by iron, nickel, cobalt, palladium, used as such, or preferably deposited on the support. 50% of the metal M as a whole must be in reduced form. It is preferable to use nickel or to associate nickel with palladium.
The Applicant has also described the use of other masses for the recovery of arsenic during this first step in the patents EP A 412 862 and WO A 90/10684.
The recovery mass for mercury used in the second step contains sulphur and a metal sulphide. The prior art process for recovery of mercury and arsenic, described in the assignee's U.S. Pat. No. 4,911,825, is carried out in the following way:
1) the hydrogen is mixed with the charge to be purified, PA0 2) the mixture is heated to the required temperature for the first reactor, thus to a temperature of between 130.degree. and 250.degree. C. for the masses described in the patent U.S. Pat. No. 4,911,825 and between 180.degree. and 450.degree. C. for the masses described in the patent WO A 90/10684. PA0 3) The mixture is placed in contact with the mass for recovery of arsenic in the first reactor where all the arsenic compounds are converted into arsine, (ASH.sub.3) and all the mercury compounds are converted into elementary mercury. The arsenic is retained by this recovery mass, but the mercury is not retained. PA0 4) The effluent From this First reactor, from which effluent arsenic has been removed, is cooled to the required temperature for the second step, to between 0.degree. and 175.degree. C. PA0 5) The cooled mixture enters the second reactor where the mercury is retained by a recovery mass for the mercury. PA0 1) the need to mix the hydrogen with the charge prior to recovery of the mercury, and PA0 2) having to cool the hydrocarbon charge between the two steps of the process. PA0 1) recovery masses (M2) for arsenic containing at least one metal from the group formed by iron, cobalt, nickel and palladium used preferably deposited on a support and in reduced form, at least for 50% of its whole, and PA0 2) recovery masses (M2) for arsenic containing at least one metal selected from the group formed by nickel, cobalt, iron, palladium and platinum, and at least one metal selected from the group formed by chromium, molybdenum, tungsten and uranium, deposited on a support.
The prior art, described hereinabove, is very effective for the recovery of arsenic and mercury which are present in various chemical forms in hydrocarbon charges. The First step permits conversion of the organomercuric species, HgR.sub.2 where R is an organic radical in the presence of hydrogen into elementary mercury and 2HR. In Fact, the conversion of organomercurics into elementary mercury is obligatory for the recovery of mercury.
Nonetheless, this prior art is ill suited to hydrocarbon charges of very low mercury content in the form of organomercuric contents. Two fundamental problems with the prior art are: